borrow checking refinements (#15290)

* added basic borrowing test

tests/effects/tcannot_borrow.nim

@@ -0,0 +1,18 @@
+discard """
+  errormsg: "cannot borrow"
+  nimout: '''tcannot_borrow.nim(16, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
+tcannot_borrow.nim(17, 3) the mutation is here
+tcannot_borrow.nim(16, 7) is the statement that connected the mutation to the parameter'''
+  line: 16
+"""
+
+{.experimental: "views".}
+
+type
+  Foo = object
+    field: string
+
+proc dangerous(s: var seq[Foo]) =
+  let meh: lent Foo = s[0]
+  s.setLen 0
+  echo meh.field